There have been widely known optical disk recording and reproducing apparatus for recording data on an optical disk and reproducing the data therefrom, using a semiconductor laser or similar devices. In such an optical recording and reproducing apparatus, the track access operation for moving the optical head to a designated track on an optical disk, consists of two sequential operations, i.e., coarse access and fine access.
More specifically, when the optical head is extremely far from a designated track position, (i) an access distance in a radial direction of an optical disk is first obtained from the distance between the present position of the optical head as detected by a position-detection sensor and the position of the designated track. Then, (ii) the optical head is moved to the designated track by a driving means such as a linear motor while the moving speed of the optical head is controlled in accordance with a predetermined speed table so that the movement of the optical head is approximately equalized to the access distance.
Thereafter, (iii) a light beam emitted from the optical head is adjusted to follow up the track by a tracking servo comprised of an objective lens actuating device, and a tracking pull-in operation (i.e. an operation for moving an optical head in close proximity to a designated track position) is completed, (iv) only an objective lens installed in the optical head is actuated so as to jump from one track to another while track numbers are being read out and (v) the optical head is thus moved to the designated track position. In this procedure, the first series of operations (i.e. the steps (i) to (ii)) is called "coarse access" and the second series of operations (i.e. the steps (iii) to (v)) is called "fine access".
In a conventional method for performing the coarse access, an optical head is actuated to access a designated track by a linear motor and its control circuit using a speed table 60 according to which the actual moving speed of the optical head is reduced at a constant acceleration as shown in FIG. 14(a). If the actual moving speed of the optical head changes in compliance with the speed table 60 faithfully, the actual moving speed perfectly coincides with the speed table 60.
There is, however, a likelihood of speed deviation (a difference between the actual speed of the optical head and the speed table) that is proportional to the acceleration in reducing the speed (negative acceleration), when employing such a speed table 60 in a general speed control system. However, the desired deceleration cannot be obtained so that the time required for stopping the optical head is prolonged and errors often occur in measuring the moving distance of the optical head. As a result, a precise tracking pull-in operation fails prolonging the time required for the tracking pull-in operation. Further, since an abrupt change often occurs in acceleration when the optical head stops, the optical head is not fixed in a steady position and the tracking servo actuated by the objective lens actuating device also becomes unstable. Although the speed deviation can be reduced by extremely increasing the loop gain of the speed control system, this is virtually impossible since the control system becomes unstable due to mechanical resonance or other problems.
To solve the above problems, the following approach has been provided: A designated track position is accessed by an optical head using a speed table 61 using acceleration to reduce the actual speed of the optical head as the optical head approaches the designated track position, as shown in FIG. 14(b). With this arrangement, it is possible to reduce the access distance without extremely increasing the loop gain of the speed control system, by means of reducing the speed deviation at the time of a tracking pull-in operation, so that the tracking pull-in operation is stabilized. An abrupt change in acceleration at the time the optical head is suspended is also prevented and, therefore, the tracking servo control can be performed in a stable condition.
In the foregoing access method, it is impossible to read out track numbers during a coarse access operation and, therefore, the following steps are taken:
(i) a present position of the optical head is detected by a position detection sensor; PA1 (ii) the number of tracks existing in a distance from the above position to a designated track position is obtained; PA1 (iii) the number of tracks thus obtained is converted to an access distance and an access operation is executed based on the access distance. However, the disadvantage remains in that the accuracy of the coarse access is often deteriorates due to variations in the accuracy of the position detection sensor and the expansion/shrinkage of the optical disk that may be caused by a change in ambient temperature. As a result, it takes more time to perform the coarse access, and the time required for performing the overall access operation is accordingly prolonged.
Further, when using the speed table 61, the access operation is performed at a lower speed as the optical head approaches the designated track so that more seek time becomes necessary. The access speed is considerably slower when the optical head is somewhere short of the designated track, so that the optical head is often halted prior to commencing a tracking pull-in operation because of friction and the like. Since the transition from coarse access to fine access is executed after the optical head reaches a predetermined allowable region proximate to the designated track, if the optical head stops before reaching the above region like in the foregoing case, the operation will proceeded to fine access.